PARIS — Weeks before Chinese authorities acknowledged that the coronavirus could be transmitted by humans, and nearly a month before the first officially recorded cases in Europe, a 42-year-old fishmonger showed up at a hospital in suburban Paris coughing, feverish and having trouble breathing. It was Dec. 27.

Now doctors in France say that the December patient may have been the earliest known coronavirus case in Europe.

If confirmed, the case of the fishmonger, Amirouche Hammar, would mean the deadly virus made an appearance on the continent long before officials there began tackling it. Such a discovery would bring a strange new wrinkle to the story of the virus in Europe, one that has the potential of blowing up the previously established chronology.

The French government says it is looking at the report. The doctors who made the finding said that they are confident in it, and that they tested the patient’s old sample twice to avoid false positives. But they acknowledged that they could not completely rule out that possibility.

The doctors also cautioned that without further analysis of the sample, it was unclear whether the man had passed the virus on to anyone else, or whether his case was tied in any way to the epidemic that arrived later.

But if the timeline of when the virus appeared in Europe does change, the official efforts to combat the contagion will turn out to have been not just too late, but hopelessly too late.

By the time the first serious measures were put in place — the French government didn’t order a lockdown until March 16 — the virus may have already appeared three months earlier, according to a study of the new case that has been peer-reviewed and accepted for formal publication in the International Journal of Antimicrobial Agents.

That, in turn, would help explain the rapidly developing catastrophe that has since unfurled in France and Europe. There have been thousands of cases, hospitalizations and deaths, in numbers that only in recent weeks have begun to abate somewhat, as a result of the French government’s rigid confinement measures.

France alone has recorded over 25,000 coronavirus deaths.

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Crowds on the grass near the Louvre on March 15. The next day, the French government ordered its lockdown.Credit...Dmitry Kostyukov for The New York Times

“If confirmed, what this case does highlight is the speed at which an infection starting in a seemingly remote part of the world, can quickly seed infections elsewhere,” said Prof. Rowland Kao, the Sir Timothy O’Shea Professor of Veterinary Epidemiology and Data Science at the University of Edinburgh, in an interview for Britain’s Science Media Centre.

“Why is this important?” he added. “It means that the lead time we have for assessment and decision-making can be very short.”

The journal publishing the report about the case has had a brush with controversy over the coronavirus, walking back a study it published about treatments for the virus. And much about this apparent first case remains a mystery.

But the authors of the paper, doctors at the Avicenne Hospital in the Paris suburb of Bobigny, among others, declare flatly: Their study is of a “patient infected with Covid-19 one month before the first reported cases in our country” whose “lack of recent travel suggests that the disease was already spreading among the French population at the end of December 2019.”

French authorities declared the first official cases of coronavirus in the country — three people who had all recently been in China — on Jan. 24. That was four days after China for the first time confirmed human-to-human transmission.

The doctors retested a sample from a patient who had suffered from pneumonia. They found the coronavirus.

“There’s no doubt for us,” said Dr. Yves Cohen, head of intensive care at the Avicenne and Jean Verdier hospitals, in the northern suburbs of Paris, and one of the authors of the study, in a telephone interview Tuesday. “It was already there in December.”

What is not clear is how the patient, Mr. Hammar, got it. Apart from a trip to Algeria last summer, he had not traveled. His wife, however, briefly exhibited some of the symptoms — coughing, principally — of the coronavirus, Dr. Cohen said.

“We’ve got some theories,” he said. “His wife had a little cough.”

Mr. Hammar’s wife, Fatiha, who works in a supermarket near Charles de Gaulle airport in Paris, told French television this week that she serves customers who come directly from the airport, “with their suitcases,” she said.

There were direct flights between that airport and the one in Wuhan, China, before borders were closed.

Experts warned that the case could not be directly tied to France’s current outbreak without a genomic analysis.

“One really has to make a distinction between the epidemic wave and isolated cases,” Samuel Alizon, an infectious diseases and epidemics specialist at the CNRS, France’s national public research organization, said in a telephone interview.

“It is quite possible,” he explained, “that there were isolated cases that led to transmission chains that died down.”

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Passengers waiting to board planes at Charles de Gaulle airport in March. The patient’s wife said she often served customers who came from the airport.Credit...Valerie Macon/Agence France-Presse — Getty Images

Mr. Alizon said it was common for epidemics imported from abroad into a given country to undergo several false starts, with transmission chains that died down on their own before one of the imported cases led to an actual epidemic.

“So the question is more: How many importation events did it take to launch the epidemic wave?” he said.

The first case outside of China was reported in Thailand on Jan. 13. But experts have long suspected that the coronavirus may have spread internationally before the first officially reported cases.

Frequently Asked Questions and Advice

Updated June 5, 2020

How many people have lost their jobs due to coronavirus in the U.S.?

The unemployment rate fell to 13.3 percent in May, the Labor Department said on June 5, an unexpected improvement in the nation’s job market as hiring rebounded faster than economists expected. Economists had forecast the unemployment rate to increase to as much as 20 percent, after it hit 14.7 percent in April, which was the highest since the government began keeping official statistics after World War II. But the unemployment rate dipped instead, with employers adding 2.5 million jobs, after more than 20 million jobs were lost in April.

Will protests set off a second viral wave of coronavirus?

Mass protests against police brutality that have brought thousands of people onto the streets in cities across America are raising the specter of new coronavirus outbreaks, prompting political leaders, physicians and public health experts to warn that the crowds could cause a surge in cases. While many political leaders affirmed the right of protesters to express themselves, they urged the demonstrators to wear face masks and maintain social distancing, both to protect themselves and to prevent further community spread of the virus. Some infectious disease experts were reassured by the fact that the protests were held outdoors, saying the open air settings could mitigate the risk of transmission.

How do we start exercising again without hurting ourselves after months of lockdown?

Exercise researchers and physicians have some blunt advice for those of us aiming to return to regular exercise now: Start slowly and then rev up your workouts, also slowly. American adults tended to be about 12 percent less active after the stay-at-home mandates began in March than they were in January. But there are steps you can take to ease your way back into regular exercise safely. First, “start at no more than 50 percent of the exercise you were doing before Covid,” says Dr. Monica Rho, the chief of musculoskeletal medicine at the Shirley Ryan AbilityLab in Chicago. Thread in some preparatory squats, too, she advises. “When you haven’t been exercising, you lose muscle mass.” Expect some muscle twinges after these preliminary, post-lockdown sessions, especially a day or two later. But sudden or increasing pain during exercise is a clarion call to stop and return home.

My state is reopening. Is it safe to go out?

States are reopening bit by bit. This means that more public spaces are available for use and more and more businesses are being allowed to open again. The federal government is largely leaving the decision up to states, and some state leaders are leaving the decision up to local authorities. Even if you aren’t being told to stay at home, it’s still a good idea to limit trips outside and your interaction with other people.

What’s the risk of catching coronavirus from a surface?

Touching contaminated objects and then infecting ourselves with the germs is not typically how the virus spreads. But it can happen. A number of studies of flu, rhinovirus, coronavirus and other microbes have shown that respiratory illnesses, including the new coronavirus, can spread by touching contaminated surfaces, particularly in places like day care centers, offices and hospitals. But a long chain of events has to happen for the disease to spread that way. The best way to protect yourself from coronavirus — whether it’s surface transmission or close human contact — is still social distancing, washing your hands, not touching your face and wearing masks.

How can I protect myself while flying?

If air travel is unavoidable, there are some steps you can take to protect yourself. Most important: Wash your hands often, and stop touching your face. If possible, choose a window seat. A study from Emory University found that during flu season, the safest place to sit on a plane is by a window, as people sitting in window seats had less contact with potentially sick people. Disinfect hard surfaces. When you get to your seat and your hands are clean, use disinfecting wipes to clean the hard surfaces at your seat like the head and arm rest, the seatbelt buckle, the remote, screen, seat back pocket and the tray table. If the seat is hard and nonporous or leather or pleather, you can wipe that down, too. (Using wipes on upholstered seats could lead to a wet seat and spreading of germs rather than killing them.)

Should I wear a mask?

The C.D.C. has recommended that all Americans wear cloth masks if they go out in public. This is a shift in federal guidance reflecting new concerns that the coronavirus is being spread by infected people who have no symptoms. Until now, the C.D.C., like the W.H.O., has advised that ordinary people don’t need to wear masks unless they are sick and coughing. Part of the reason was to preserve medical-grade masks for health care workers who desperately need them at a time when they are in continuously short supply. Masks don’t replace hand washing and social distancing.

What should I do if I feel sick?

If you’ve been exposed to the coronavirus or think you have, and have a fever or symptoms like a cough or difficulty breathing, call a doctor. They should give you advice on whether you should be tested, how to get tested, and how to seek medical treatment without potentially infecting or exposing others.

The detection of the potential new case in France follows similar instances in the United States, where officials recently discovered that deaths from the virus had occurred weeks earlier than previously known, and a model suggested that silent outbreaks had spread for weeks before detection.

The French government has said very little about the case so far.

France’s health ministry said on Tuesday that authorities were in contact with scientists and experts from other countries on the timeline of the spread of the virus, and that they would carry out further investigations “if they appear necessary.”

“We are in permanent contact with our European and Chinese counterparts on the issue, in order to better understand the spread of the virus at the global level,” the ministry said.

Dr. Olivier Bouchaud, an infectious disease specialist at the same hospital as Dr. Cohen, told the LCI news channel on Tuesday that it was common to keep frozen samples from patients with lung infections for later testing.

“It isn’t very surprising,” Dr. Bouchaud said of the Dec. 27 positive case, noting that in China the virus also circulated under the radar for weeks before the first official cases were detected.

Frédéric Keck, a biosecurity expert at CNRS, said, “If Covid existed in November” — which some reports suggest was the case — “it is certainly possible that it was here in December.”

“We never really know when an epidemic starts,” Mr. Keck said.

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The Place de la Concorde, one of the major public squares in Paris, was nearly empty last month.Credit...Dmitry Kostyukov for The New York Times

Mr. Hammar, who lives in Bobigny, a northern suburb of Paris, said in an interview with BFM TV that he drove himself to the emergency ward at 5 a.m. on Dec. 27 after several days of coughing, difficulty breathing and chest pains.

Mr. Hammar, who has a history of asthma and diabetes, was diagnosed with a pulmonary infection but quickly recovered and was discharged two days later.

“I was surprised, given the devastation that the illness is causing,” Mr. Hammar said of learning, months later, that he had been tested positive for Covid-19.